The present invention relates generally to the field of satellite communication systems. More specifically, the present invention relates to embodiments of satellite communication systems suited to low data volume communications.
A conventional Mobile Satellite System (MSS) can be configured to provide services, such as voice and packet data communication, throughout the world. Referring now to FIG. 1, a typical MSS 100 comprises one or more geostationary satellites 102, one or more Gateway Stations (GS) 104, and one or more Satellite Terminals (ST) 106. The STs 106 can include mobile terminals (handsets), vehicle terminals, and/or fixed terminals. The GS 104 can be configured with external interfaces to existing fixed telecommunication infrastructure as well as to the wireless telecommunication infrastructure. For example, a GS 104 may interface to a Public Switched Telephone Network (PSTN) 108. The subsystems in Gateways can be oriented to various types of transmission functionality, e.g., circuit-switched or packet-switched. The names of the subsystems vary between implementations. The term for all the ground-based subsystems is Network Infrastructure 110, which includes the GS and PSTN subsystems in FIG. 1. The satellite directs energy in the forward link to areas on the ground called beams 112. The same concept of beam-forming is applied in the return link to separately capture the signals from terminals in each beam at the satellite.
Information is communicated in finite duration transmissions called bursts. Bursts are composed of: waveforms related to physical layer functions such as detection and synchronization (e.g., pilot signals); and waveforms that contain modulated data. The modulated data includes payload fields and error detection fields (e.g., CRC). The payload fields may contain control information (such as terminal identity), and application-related information. Any payload information that is not application-related is defined as an overhead.
Information can be transmitted via these satellites 102 using a Common Air Interface (CAI). Existing satellite CAIs typically concentrate on efficient operation for relatively large quantities of data. For example, a voice call lasting one minute might involve 30 kB (kilo-Bytes) or more of information transmission in each direction. Packet data operations often involve even larger quantities of data, frequently in the MB (Mega-Byte) range. Providing a connection in a conventional MSS typically involves a sequence of steps including:                Requesting and establishing a link between a ST and network infrastructure via a satellite;        Exchanging information characterizing the capabilities of the end points;        Exchanging information describing the objectives and configuration of the connection;        Transmitting the data and related acknowledgements; and        Exchanging information to terminate the connection.        
Prior to transferring information, an ST 106 typically must “register” with the network. In addition, the ST 106 typically must “re-register” when it moves from one satellite beam to another. When an ST is registered, the network infrastructure is aware that the ST is present, and the beam within which that ST can be located. After a ST is registered, data exchanges can proceed. A conventional MSS data exchange may start with establishing a communication channel. This may include sending a Random Access Channel (RACH) burst from a ST 106 to a satellite 102, which passes the RACH burst to a gateway 104. The RACH burst might include source information, such as a called party, terminal identity (ID), terminal capabilities, the message intent (such as establishing a packet connection) and possibly location information. Next an Access Grant Channel (AGCH) burst may be sent from the gateway 104 to the ST 106 to establish a bidirectional traffic channel for further exchange of information. A typical AGCH burst can provide other information, such as an indication of available resources for the ST 106. Security information may be exchanged back and forth between the gateway 104 and ST 106. Further capability information, such as maximum data rate, may also be exchanged between the gateway 104 and ST 106.
After a communication channel is established, data may be sent between the ST 106, satellite 102, and gateway 104. The data may be sent in multiple messages. Each message includes header and protocol overhead, which will vary in quantity depending on the scenario, and can amount to approximately 20% of the message. Acknowledgement messages (ACK) are also sent to acknowledge the successful receipt of the data messages. If, the data messages are not successfully received, a Non-Acknowledgement message (NACK) is sent and the data message(s) are resent.
After data communications are complete, the ST 106 will send a “done” message to the satellite 102 which gets passed on to the gateway 104 and, if the “done” message is successfully received, a termination message is sent to the ST 106 acknowledging receipt of the “done” message. For large quantities of data, the exchanges other than “transmitting the data”, can correspond to a reasonable overhead. However, for smaller data exchanges, the overhead can significantly impact efficiency.